The present invention relates to pulse propagation analysis and, more particularly, to the analysis of one of two intersecting electrical pulses to determine whether a surface anomaly exists at the location where the two pulses intersect.
Pulse propagation analysis has long been proposed as a tool for nonintrusively detecting anomalies in elongate, conductive members such as buried cables and the like. In U.S. Pat. No. 4,970,467, the present applicant proposed propagating two electrical pulses along a conductor such that they intersect at a predetermined location along the conductor. By analyzing one of these pulses after they have passed through the predetermined location, the applicant found that the presence or absence of an anomaly at the predetermined location could be predicted.
The basic principle disclosed in the ""467 patent was also used in U.S. Pat. Nos. 5,189,374, 5,243,294, and 5,270,661, all also issued to the present applicant. The ""374 patent applied the basic principles described in the ""467 patent in the context of a conductive member, such as a well casing, where one end of the member is more accessible than the other end. The ""294 patent teaches that the propagation delay for the length of the conductive member under test can be used to set the timing of the pulses such that the pulses intersect at a plurality of desired locations along the portion of the conductive member under test. The ""661 patent teaches applying a DC electrical current to the member to provide an electrical potential. Changes in the electrical potential are observed and correlated with analysis of pulses that have intersected along the member.
While the basic principles described in the foregoing patents are generally applicable to any conductive member, the inventions claimed in those patents were, for illustration purposes, disclosed in the context of a buried pipe. In contrast, the present invention is of particular significance when used in the context of detecting surface anomalies, such as corrosion, on insulated pipe located above ground. Accordingly, that application will be described in detail herein. The present invention may, however, have broader application to detecting other anomalies on elongate, conductive members. The scope of the present invention should thus be determined not based on the following detailed description but instead on the claims appended hereto.
In manufacturing facilities such as oil refineries and the like, miles of pipe are used to carry fluids being processed or used in the refining process. The failure of such pipes can cause extensive damage, and these pipes are often routinely inspected to avoid pipe failures and the damage resulting therefrom. For a variety of reasons, such as energy conservation and worker safety, many of these pipes are wrapped with insulation.
For any given pipe, the entire interior surface of the pipe is subject to essentially the same conditions. The interior of the pipe thus can be satisfactorily tested by sampling analysis of the interior surface at discrete points and using statistical analysis to draw inferences regarding the condition of the pipe at locations between the sampled points.
The exterior surface of insulated pipe cannot be adequately tested using sampling and statistical inferences, however, because one cannot assume that the entire exterior surface of insulated pipe is subject to the same conditions. To the contrary, anomalies on the exterior surface of a pipe tend to be localized and caused by factors specific to that location.
Accordingly, to inspect the exterior surface of insulated pipes, the insulation must be removed and the exterior surface visually inspected. The process of removing and reinstalling this insulation is very expensive. In this context a buried pipe will be considered one form of insulated pipe, as buried pipe cannot be visually inspected without expensive excavation. Accordingly, the need exists for a method of testing the condition of the exterior surface of an insulated pipe by means other than visual inspection after the insulation has been removed.
From the foregoing, it should be apparent that one specific object of the present invention is to provide methods and apparatus that allow a surface anomalies in a length of pipe to be detected and located.
Another more specific object of the present invention is to provide pulse propagation systems and methods having a favorable mix of the following characteristics:
(a) allows unobtrusive testing of elongate conductive members such as pipes and the like;
(b) obviates the need to visually inspect pipe such as insulated pipe that is not easily visually inspected; and
(c) can be effectively and consistently applied in a cost efficient manner.
By empirically testing under controlled conditions sections of pipe that are identical accept for the fact that one pipe has surface corrosion and the other pipe does not, the applicant has determined that corrosion on the surface of a pipe yields a measurable change in propagation velocity, rise time, and amplitude of an electrical pulse passing through a section of pipe having surface corrosion. While these parameters will vary somewhat even in good pipe, the variations are noticeably more severe when the pipe has surface corrosion thereon.
The applicant thus concluded that the effect of corrosion on propagation of velocity, rise time, and amplitude of single pulse should also be observable in some form in a pulse that has intersected another pulse at a predetermined location. By looking for the affects of corrosion in a two-pulse system, not only can corrosion be identified but it can also be located and significantly simplifies the testing process.
The present invention in its most basic form is thus a method of detecting an anomaly of conductive member comprising the steps of sending an electrical pulse along the conductive member and analyzing characteristics of the pulse such as propagation speed, rise time, waveform shape, and amplitude to ascertain whether corrosion exists on the conductive member. By comparing these characteristics to those generated for a known good pipe, the absence or presence of an anomaly can be predicted.
In another exemplary form, the present invention is a method of detecting corrosion on the surface of a pipe. In this situation, it is not only desirable to detect the presence or absence of an anomaly, but to ascertain the location of the anomaly along the length of pipe.
To allow an anomaly not only to be detected but to be located, the present invention comprises the steps of sending two electrical pulses along the conductive member such that they intersect at an intersecting location and analyzing at least one characteristic of at least one of the pulses after they have passed through the intersecting location to determine whether an anomaly exists at the intersecting location. To provide information related to corrosion, the pulse characteristic that is analyzed should provide an indication of localized velocity changes of the pulse along the given section of pipe. To this end, such factors as the propagation delay, rise time of the leading edge, and amplitude of the pulse may be considered to determine whether corrosion exists.
To provide a complete picture of an entire length of pipe, a plurality of pulses are timed to intersect at different predetermined locations along the length of pipe. Then, one pulse associated with each predetermined location is analyzed to determine whether an anomaly exists at the predetermined location.
While a combination of characteristics of a given pulse may be analyzed to determine corrosion, the applicant has found that the most observable, with currently available test equipment limitations, indications of surface anomalies are contained at or near the leading edge of the pulse. The rise time of the leading edge, the amplitude of the pulse adjacent to the leading edge, the shape of the leading edge as compared to the shape of the leading edge for pulses generated at known good locations, and whether the leading edge is shifted from a predicted position in time all may indicate the presence or absence of surface anomalies at the intersecting location.
In practice, the applicant has found that electrically connecting the points on a conductive member at either end of the section of interest removes certain variables relating to grounding and substantially reduces the amount of power required to generate a given pulse. By referring all test equipment to a reference potential set at the potential of the electrically connected end points, very low power pulses may be used. Low power pulses appear to provide more meaningful information related to the surface of the pipe than do higher power pulses.
Additionally, the applicant has found that, especially for short lengths of pipe, the widths of the pulses used should be much greater than the propagation delay between the end points of the portion of the conductive member in question. By selecting a pulse width at least ten times the length of the propagation delay, the leading edge of the pulse is isolated from the trailing edge thereof. This is because for short pipe lengths, equipment limitations in the development of short duration pulse widths can result in ringing at the leading and trailing edges which obscure the effects on the pulse waveform caused by surface anomalies.
As generally described above, the present invention results in an unobtrusive test for exterior corrosion that can be performed with high resolution along a length of covered pipe simply by providing access to the end points of the section of pipe in question. The present invention thus obviates the need to remove insulation or dig up pipe for the purpose of visually inspecting the exterior thereof. Other objects and advantages of the present invention will become apparent from the following detailed description.